The prior art, as shown in FIG. 7, discloses a single phase triangle wave fed to each pulse width modulator of a three phase drive. The result is that all upper transistors can turn on during 1/2 of the pulse width modulator cycle, and all lower devices can turn on during the other 1/2 cycle. This results in no current flow and no conduction and no gain when the command is zero. A large three phase command causes a condition where current is always flowing in a winding (continuous conduction) and results in full gain. In between is discontinuous conduction where the gain varies from zero to maximum. FIG. 8 shows a graph of the gain of the motor through zero command with respect to the prior art.
Discontinuous conduction causes a stability problem in high bandwidth servos by reducing gain and bandwidth. See IEEE Transactions on Industry Applications, Vol. 1A-21, No. 1, January/February 1985. The present invention allows for continuous conduction at all times (even zero command), by providing a phase difference between oscillator signals provided to each pulse width modulator. The result is that gain is held constant through zero command because no discontinuous conduction occurs.